Lithography is a printing system taking advantage of essential mutual repulsion between water and oil. Surfaces of lithographic printing plates comprise areas which accept water and repel a greasy ink and areas which repel water and accept a greasy ink, the former areas corresponding to non-image areas and the latter areas corresponding to image areas. Therefore, a light-sensitive composition which can be used in the production of lithographic printing plates is required to have a water repelling property and a greasy ink accepting property after image formation.
The light-sensitive compositions used for the production of lithographic printing plates are classified into positive working compositions and negative working compositions. The positive working light-sensitive compositions generally comprise o-quinonediazide compounds.
Positive working presensitized lithographic printing plate precursors comprise an appropriate support, such as a metal, a plastic, etc., generally having coated thereon an o-quinonediazide compound alone or a mixture of an o-quinonediazide compound and an alkali-soluble resin, such as a novolak including a phenol-formaldehyde resin, a cresol-formaldehyde resin, etc. When the printing plate precursor is exposed to actinic light through a positive transparency, the o-quinonediazide compound in the exposed areas is decomposed to form an alkali-soluble substance, which can be easily removed by an alkaline aqueous solution to produce a positive image. Therefore, if using a support having a hydrophilic surface, the hydrophilic surface of the support in the areas from which the o-quinonediazide compound has been removed by an alkaline aqueous solution is exposed and accepts water but repels a greasy ink. To the contrary, the non-exposed areas remain oleophilic and accepts a greasy ink.
On the other hand, negative working light-sensitive compositions generally employ diazonium salts, azide compounds, or photopolymerizable compounds. Such compounds are coated on a support individually or in combination with appropriate additives such as resins. When a support having a hydrophilic surface is used, the non-exposed areas are removed with a developing solution to expose the hydrophilic surface of the support. The thus exposed areas accept water and repel an ink. To the contrary, the areas which have been cured upon exposure to light remain oleophilic and accept an ink.
When the thus produced lithographic printing plates are mounted on an offset printer, high quality prints can be obtained therefrom. Lithographic printing plates produced from the above-described presensitized printing plate precursors can generally produce several tens of thousands clear prints per plate by properly selecting a support and a composition for a light-sensitive layer provided thereon. In particular, use of an aluminum sheet that has been subjected to graining and anodic oxidation as the support makes it possible to obtain about 100,000 high quality prints per plate.
However, there has been a demand for further increasing the number of prints produced per printing plate, i.e., press life. Such a demand can effectively be satisfied by a process in which a presensitized printing plate precursor comprising a metal support, e.g., aluminum, zinc, etc., is exposed to light and developed in a conventional manner, and thereafter, heated at a high temperature (a so-called burning-in treatment) to reinforce the image areas. By the burning-in treatment, the printing durability (or "press life") of lithographic printing plates can be increased several times.
Further, when printing is carried out using a special printing ink containing a large amount of a component that dissolves image areas on the printing plate, such as an ultraviolet-curing ink or a low temperature-drying ink, the printing plate produced by an ordinary process omitting a burning-in treatment suffers intense elution on the image areas, resulting in considerable deterioration of press life. On the other hand, a printing plate that has been subjected to burning-in treatment has a greatly improved solvent resistance of image areas and, therefore, can produce a satisfactory number of prints even with the above-described special printing inks.
However, the burning-in treatment tends to deteriorate the hydrophilic property of the non-image areas, i.e., the areas wherein the hydrophilic surface of the support is exposed by development, and renders the non-image areas receptive to printing ink, which leads to formation of stains on the background of prints. If the heating of the burning-in treatment is effected only to such an extent that it does not cause any such background stains, a burning effect sufficient to achieve the desired reinforcement of image areas cannot be attained. Therefore, in order to prevent formation of stains in non-image areas due to the burning-in treatment, a counter-etching treatment should always be carried out before and/or after the burning-in treatment.
Various counter-etching treatments have been proposed. For example, a processing solution which can be used after burning-in treatment for removing the scum in the non-image areas to restore the hydrophilic surface includes an aqueous solution of a fluoride, e.g., hydrofluoric acid, borofluoric acid, hydrosilicofluoric acid, and the like. These fluorides, however, are generally very poisonous substances, and give rise to many problems from the standpoint of environmental pollution.
Furthermore, the above-described counter-etching treatment with a fluoride aqueous solution corrodes a metal surface of the support. Since a corroded metal surface easily takes scratches and has poor abrasion resistance, the property of non-image areas to accept water (i.e., water retention property) is lost, causing background stains during printing, thereby reducing press life.
Further, a treatment for use before the burning-in treatment for preventing the scumming of non-image areas includes a treatment with an aqueous solution of a salt of an organic sulfonic acid, e.g., a sodium alkylnaphthalenesulfonate and a sodium alkyldiphenyl ether sulfonate, or lithium nitrate before burning-in treatment, as described in U.S. Pat. No. 4,294,910. Of these processing solutions, an aqueous solution of the organic sulfonate causes remarkable foaming and is, therefore, unsuitable for use in a commonly employed processing machine of spray circulatory system and also cannot be applied to automation of plate making. On the other hand, treatment with a solution containing lithium nitrate followed by burning-in treatment is not sufficient to prevent background stains. Furthermore, U.S. Pat. No. 4,063,507 discloses a method comprising treating a printing plate precursor with an aqueous solution containing sublimating boric acid and a salt thereof followed by burning-in treatment in the presence of these compounds, but this method is also insufficient in prevention of background stains. In particular, depending on conditions for washing with water, gumming, etc., subsequent to the burning-in treatment, i.e., if washing with water is insufficient or a desensitizing gum comprising a dextrin (which has a poor activity to provide a hydrophilic property) is used at a gumming step, scumming is apt to generate. Still further, Canadian Pat. No. 1,084,758 discloses a treatment before burning-in treatment with an aqueous solution of a water-soluble organic substance, e.g., gum arabic, cellulose ethers, polyacrylic acid, etc., and/or a water-soluble inorganic salt, e.g., nitrates, phosphates, sulfates, halides of alkali metals or alkaline earth metals, etc. However, this method is not only insufficient to achieve complete prevention of scumming but also disadvantageous in that image areas hardly accept printing ink (i.e., image-blinding) in carrying out printing, particularly when an aqueous solution of a water-soluble polymer, such as gum arabic, polyacrylic acid, etc., is used as a desensitizing gum. Moreover, U.S. Pat. No. 4,355,096 discloses a treatment before burning-in treatment with an aqueous solution containing an amine having a carboxyl group, e.g., ethylenediaminetetraacetic acid, a hydroxyalkylethylenediaminetriacetic acid, etc., or a salt thereof. This method is also insufficient in preventing scumming.